Magnetically shielded electrodeless light source

ABSTRACT

A dimmable electrodeless light source includes an electrodeless lamp, an electronic ballast and a dimming module. The light source further includes coupling transformers coupled to the electrodeless lamp for inductively coupling power to the lamp to generate light. An auxiliary winding electromagnetically coupled to the primary winding of at least one of the coupling transformers is driven by switching circuitry in the dimming module. The switching circuitry is pulse width modulated to control the average brightness of the light generated by the electrodeless lamp. An exemplary application for the dimmable electrodeless light source is as a backlight for a video display device, such as a liquid crystal display unit. The dimmable electrodeless light source further includes a magnetic shield device that is operably positioned with respect to the electrodeless lamp. The magnetic shield device produces a magnetic field that substantially opposes, and cancels, the magnetic field that is produced by the electrodeless lamp when energized. In an alternative embodiment, the magnetic shield device produces a magnetic field which, when combined with the lamp magnetic field, results in a total magnetic field that is substantially constant regardless of the energization level of the lamp (e.g., totally energized or dimmed). The magnetic shield thus reduces visual artifacts that might otherwise appear on a video display unit due to a variation of the magnetic field produced by the lamp.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The present invention relates generally to electrodeless light sources and, more particularly, to a method and apparatus for shielding electromagnetic interference generated by an electrodeless fluorescent light source for a backlit display.

2. Background of the Related Art

This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

Conventional fluorescent lamps are driven with an electronic ballast which powers the lamps via electrodes disposed at each end of the lamp. The electrodes, however, are major life-limiting components of the fluorescent lamp. Electrodeless fluorescent lamps also are known. An electrodeless lamp is configured as a closed loop tube around which one or more coupling transformers are positioned. As with conventional fluorescent tubes, the electrodeless lamp is energized by an electronic ballast. However, rather than applying power to electrodes disposed at each end of a lamp tube, the ballast drives the coupling transformers, which, in turn, inductively couple the power to the lamp. The elimination of electrodes from the fluorescent lamp is particularly advantageous as it increases the life and reliability of the lamp and systems incorporating such lamps. Thus, electrodeless lamps are particularly useful in applications in which access to the lamps is restricted such that replacement of the lamps becomes difficult or expensive.

Backlit video display devices are one type of application in which the access to the lamp is not readily available. Such video displays may be found in computer systems, automatic teller machines, information kiosks, gas pumps, shipboard controls, etc. To enhance viewing of displayed images, such video displays commonly include a backlight source to provide a brightly lit background that contrasts with the displayed image. However, such video displays often are located in environments in which the ambient lighting conditions vary considerably, interfering with vivid viewing of the displayed image. For example, in a dimly lit environment (e.g., a cloudy day, the enclosed interior of a ship, etc.), a brightly lit background provides for the best viewing of a displayed image. However, in a brightly lit environment (e.g., a sunny day, a well-lit office, etc.), a dimly lit background provides for better viewing. Unfortunately, electrodeless lamps rarely are used in such displays due to the lack of suitable means for dimming such lamps. Accordingly, it would be desirable to provide the capability to control the brightness of the backlighting to compensate for variations in ambient lighting to enhance the viewing capabilities of the video display unit further.

The use of electrodeless fluorescent lamps is not limited to backlight sources for video display units, such as a liquid crystal display (LCD), or applications in which the lamp is not readily accessible. However, when used as a backlight source, the inventor has noted that video artifacts (e.g., dark horizontal stripes or bars) may appear on the display when the lamp is energized. The artifacts are believed to result from the effects of a stray magnetic field generated by the AC lamp current when the lamp is energized. These artifacts, which may detract from or otherwise impair viewing of an image, are particularly noticeable when dimming circuitry is incorporated into the display system.

Dimming of such display systems generally may operate via the implementation of circuitry which varies the energy provided to the electrodeless lamp. For example, such circuitry may energize the lamp at an adjustable duty cycle using a pulse width modulation scheme. As a result of the pulse width modulation, the AC current through the electrodeless lamp flows in bursts and generates corresponding bursts of the stray magnetic field. The inventor has observed that when the lamp is positioned near the display panel during the time interval when the AC lamp current is flowing, video artifacts on the display panel (e.g., dark horizontal stripes) are particularly visible. Accordingly, it would be desirable to provide an electrodeless lamp source for a backlit display system including a feature which shields the display panel from the magnetic field generated when AC current is flowing in the electrodeless lamp, particularly for a system which incorporates a dimming feature.

The present invention may address one or more of the problems set forth above.

SUMMARY OF THE INVENTION

Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.

In accordance with one aspect of the present invention, an electrodeless light source includes an electrodeless lamp which generates a lamp magnetic field when energized, and a shield device disposed proximate the electrodeless lamp and configured to oppose the lamp magnetic field.

For example, in an exemplary embodiment, the shield device comprises a closed conductive loop. The conductive loop is positioned with respect to the lamp such that the lamp magnetic field causes a shield current to flow in the closed loop. The shield current, in turn, causes generation of a shield magnetic field substantially opposes the lamp magnetic field.

In accordance with another aspect of the present invention, a backlit display device includes an electrodeless lamp to generate light, a power source to energize the lamp, a display unit to display an image, and a shield device. The lamp is operably positioned with respect to the display unit to illuminate the display unit to enhance viewing of the image. When the lamp is energized, AC lamp current flows therethrough and causes generation of a lamp magnetic field that may cause effects, such as visual artifacts, that may impair viewing of the image on the display unit. The shield device is operably positioned with respect to the lamp and the display unit to reduce the effects of the lamp magnetic field on viewing of the image.

In accordance with a further aspect of the present invention, there is provided a method for reducing an effect of a first magnetic field on a display unit having an electrodeless backlight source. The backlight source includes an electrodeless lamp that produces the first magnetic field which the lamp is energized. The method comprises inductively coupling energy to the lamp such that an AC lamp current flows in the lamp when the lamp is energized and generating light. The method further includes generating a second magnetic field that substantially opposes the first magnetic field. In an exemplary embodiment, generating the second magnetic field is accomplished by inducing a shield current in a shield device that is operably positioned with respect to the lamp such that the second magnetic field substantially opposes the first magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which like reference numerals denote like elements, and:

FIG. 1 is a block diagram of a dimmable light source including an electrodeless lamp in accordance with the present invention;

FIG. 2 is a diagrammatic illustration of an embodiment of the dimmable light source of FIG. 1 in which the dimming circuitry includes an auxiliary winding coupled to one of the coupling transformers which energizes the lamp;

FIG. 3 is an exemplary, simplified schematic diagram of the relationship between the windings of the coupling transformers, the ballast and the dimming circuitry of FIG. 2;

FIG. 4 is an electrical schematic of an exemplary embodiment of the dimming circuitry of the dimmable light source of FIG. 2;

FIG. 5 illustrates the assembly of the dimmable light source of FIG. 2 with an exemplary magnetic shield device, showing the physical connections between the electrodeless lamp and coupling transformers, the dimming module, and the ballast as well as an exemplary positional relationship between the shield device and the lamp;

FIG. 5A illustrates a partially exploded view showing the assembly of a magnetic shield to an electrodeless lamp, in accordance with an exemplary embodiment;

FIG. 6 illustrates the assembly of the shielded light source of FIG. 5 in a video display device for a backlighting application, showing an exemplary positional relationship of an exemplary magnetic shield with respect to the electrodeless lamp and the display panel of the video display device;

FIG. 7 is a functional block diagram of a backlit display system incorporated in the video display device of FIG. 6;

FIG. 8 is an electrical schematic diagram of the relationship between another exemplary magnetic shield device and the electrodeless lamp assembly;

FIG. 9 is a perspective view of an electrodeless lamp, showing the plane of the electrodeless lamp and the vector of the magnetic field generated by the lamp; and

FIG. 10 is an electrical schematic diagram of another exemplary relationship between a magnetic shield device and the electrodeless lamp assembly.

DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

Turning now to FIG. 1, a block diagram of a dimmable light source 10 is illustrated. The dimmable light source 10 includes an electrodeless lamp 12, an AC power source 14 (e.g., a ballast), and a dimming module 16. The ballast 14 includes circuitry configured to provide energy to the electrodeless lamp 12 to cause the lamp 12 to generate light. The dimming module 16 includes circuitry configured to control the amount of energy provided to the lamp 12 to control the brightness of the generated light.

By way of example, the electrodeless lamp 12 can be an inductively coupled electrodeless fluorescent lamp, such as a lamp included in a lamp assembly available from OSRAM SYLVANIA Products, Inc., located in Danvers, Mass., under one of the product names, ICETRON™ 100 and ICETRON™ 150, which are described in the SYLVANIA ICETRON Design Guide, Document No. FL022 07/98, and which are 100W and 150W systems, respectively. Such lamps are configured as sealed, closed loop vessels, which use electromagnetic-induction technology to energize the lamp and generate light. Each lamp may be made of a hollow glass tube that is bent onto itself in a closed, rectangular configuration. The inside wall of the vessel is coated with fluorescent paint and the inner volume filled with a mixture of gases and mercury vapor. The lamp generates light when the voltage on the tube is sufficiently high to ignite the interior gas. When the lamp is thus energized, an AC lamp current flows within the tube.

Referring to FIG. 2, the lamp 12 is energized by an electromagnetic field produced by a pair of coupling transformers 18 and 20. The coupling transformers 18 and 20 are driven by the electronic ballast 14, such as the QUICKTRONIC® I.C.E. ballast available from OSRAM SYLVANIA which operates at a frequency of 250 kHz, or any other suitable electronic ballast. The ballast 14 receives input power from a conventional 120VAC power line via a power plug 22.

The coupling transformers 18 and 20 are substantially identical transformers, each of which include a respective ferrite core 24 and 26, a respective primary winding 28 and 30 and a respective secondary lamp winding 32 and 34. The secondary lamp windings 32 and 34 comprise the closed loop lamp vessel which threads through the cores 24 and 26. The SYLVANIA ICETRON lamp assembly, for instance, includes both the lamp and the coupling transformers. The coupling transformers 18 and 20 advantageously have split cores, so that the transformers 18 and 20 may be disposed about the lamp tube and retained by clamps which secure the two halves of each core together, as will be discussed in further detail below. The interconnections of the windings of transformers 18 and 20 are illustrated in FIG. 3.

Referring to FIG. 3, the primary winding 28 of the transformer 18 and the primary winding 30 of the transformer 20 are driven by the ballast 14. The primary winding 28 is connected in parallel with the series combination of a resistor 36 and the primary winding 30. The secondary lamp winding 32 of the transformer 18 and the secondary winding 34 of the transformer 20 are connected in series. The primary windings 28 and 30 are driven by the ballast 14 and electromagnetically couple energy from the ballast 14 to the secondary lamp windings 32 and 34, respectively. The secondary windings 32 and 34, which are provided by the lamp vessel, couple the energy to electrodeless lamp 12 to cause the lamp 12 to generate light and an AC lamp current to flow within the vessel. In this exemplary embodiment, the primary windings 28 and 30 each are eighteen turns of magnet wire, and each secondary winding is one turn of the lamp vessel. Accordingly, the turns ratio of the overall magnetic circuit is 18:2 (i.e., 9:1) in this exemplary embodiment.

Referring again to FIGS. 2 and 3, the resistor 36 is connected in series with the primary winding 30 of the coupling transformer 20. The resistor 36 is sized to present a minimum load impedance to the ballast 14 and, in one embodiment, has a value of 50 ohms. A minimum load impedance is desirable because conventional ballasts typically include protection circuitry which interrupt operation of the ballast upon detection of load changes. For example, a ballast may include a protection circuit to interrupt operation if a “no load” condition is detected. Further, the ballast electronics may include a protection circuit to interrupt operation if a short circuit condition on the ballast output is detected. Accordingly, the connection of the resistor 36 in series with the output of the ballast 14 ensures that the operation of the ballast electronics shall not be disturbed by the inclusion and/or operation of the dimming circuitry.

Referring to FIGS. 2 and 3, the transformer 20 also includes an auxiliary winding 38. In the disclosed embodiment, the auxiliary winding 38 is made of four turns of magnet wire disposed about the core 26 of the transformer 20. The auxiliary winding 38 is connected to a switch 40 in the dimming module 16. The module 16 further includes a drive device 42 for transitioning switch 40 between alternating conducting and non-conducting states. In the conducting state, a current-carrying path is established through the auxiliary winding 38 and the switch 40. In the non-conducting state, the current-carrying path is interrupted.

The switch 40 can be any type of switching device capable of alternating between conductive and non-conductive states when driven by a drive device. For example, the switch 40 can be a cam-driven switch that is mechanically operated by a multi-lobed cam driven by a rotating shaft. The cam-driven switch can include mechanical provisions for varying the percentage of time that the switch is closed during each rotation cycle (i.e., the duty cycle). Alternatively, the switch 40 can be one or more switching transistors which are driven by appropriate electronic drive circuitry at a selected switching frequency. The electronic drive circuitry can include electrical provisions for varying the percentage of time that the transistor or transistors are closed during each frequency cycle.

Regardless of whether the switch is mechanically driven or electrically driven, when the switch is closed, current flows through the switch and the auxiliary winding to create a short circuit. The short circuit condition is reflected onto the primary winding of the coupling transformer and prohibits, or substantially restricts, the inductive coupling of energy to the lamp. The average brightness of light generated by the lamp during each switching cycle (or shaft rotation) can thus be varied by adjusting the duty cycle of the switch. In the embodiments disclosed, the average brightness increases as the duty cycle of the switch (i.e., the percentage on-time) is decreased. Conversely, the average brightness decreases as the duty cycle of the switch is increased.

In the embodiment disclosed in FIG. 2, the duty cycle of the switch 40 can be adjusted via a brightness adjustment device 44 coupled to the dimming module 16. The device 44 can be a potentiometer having a variable impedance, for instance. The brightness adjustment device 44 advantageously is accessible to a user of the dimmable light source 10 and can include a panel-mounted control device, such as an adjustment knob, dial, or the like. In other embodiments, the brightness adjustment device 44 may operate without user action by including a photodetector which detects ambient lighting conditions and provides an electrical signal representative of the lighting condition for example. The dimming module 16 can be configured to adjust the duty cycle of the switch 40 in response to the electrical signal. The duty cycle may be adjusted, for instance, in discrete steps to provide for discrete brightness levels within a dimming range. Alternatively, the duty cycle may be continuously adjusted to provide for continuous variation of the brightness of the light over the dimming range. The relationship between an exemplary brightness adjustment device 44 and the electronic circuitry of the dimming module 16 will be explained in further detail below with reference to the schematic diagram of FIG. 4.

Referring again to FIG. 2, the dimming module 16, which may be powered by a conventional auxiliary power supply 46 that converts a 120VAC input to a 12VDC output, may be connected to a frequency adjustment device 48. Device 48 can include a potentiometer having a panel-mounted, user-accessible control device (e.g., an adjustment knob or dial) that provides for adjustment of the switching frequency of the switch 40. The frequency adjustment feature is particularly advantageous when the lighting source 10 is used as a backlight for a video display because adjustment of the switching frequency of the switch 40 can eliminate visual artifacts that may be visible on the video display due to electromechanical, electrical and/or optical coupling effects caused by the switching of the dimming circuitry. For example, the user of a dimmable light source or of a backlit video display device may perceive a flicker effect in the lighting that is caused by the interruption of generated light by the dimming circuitry. To eliminate the flicker, the switching frequency may be adjusted to a rate that is sufficiently fast such that the flicker cannot be perceived by a user. It has been found that a switching frequency of approximately 120 Hz is particularly suitable to avoid flicker.

Adjustment of the switching frequency of the switch 40 may also be desirable to synchronize the switching frequency with the frequency of the vertical refresh video signal of a video display unit. In some instances, if the switching frequency is not synchronized with the vertical refresh rate, the user may perceive visual artifacts on the display, such as scrolling lines. A panel-mounted control device for varying the switching frequency can allow the user to substantially eliminate the undesirable video effects such that viewing of an image of the display unit is minimally impaired.

A manually controlled, user-accessible switching frequency adjustment device is optional. For example, the switching frequency adjustment device 48 can be an automatic device or circuit that automatically adjusts the switching frequency in response to a particular parameter. For instance, in an embodiment of the invention in which the dimmable light source 10 is installed as a backlight in a video display device, the frequency adjustment device 48 can be replaced with a frequency synchronization circuit, such as a conventional phase locked loop, that has an input for receiving the vertical refresh video signal of the display unit. In such an embodiment, the frequency synchronization circuit can automatically synchronize the switching frequency to the vertical refresh rate. Regardless whether the switching frequency is automatically or manually adjusted, the adjustment may be performed in discrete steps over a range of frequencies, or the adjustment may be continuous over the range. In other embodiments of the invention, the switching frequency can be set at a fixed frequency (e.g., approximately 120 Hz) which is known to eliminate or minimize the occurrence of many visual artifacts.

Turning now to FIG. 4, a schematic of an exemplary electronic embodiment of the dimming module 16 is illustrated. It should be noted that the following description focuses on the functions of the primary components of the dimming module and does not discuss in detail the interconnections or the specific function of each individual electrical component illustrated in the schematic, as such details are conventional and would be clearly understood by any person of ordinary skill in the art who reviews this description and the accompanying FIGURES. Further, it should be understood that the specific circuitry illustrated is merely one example of a dimming module for adjusting the brightness of light generated by an electrodeless lamp. It is currently believed that the functions performed by the various electrical devices could be performed by other conventional devices arranged in other configurations, as would be well known by any person of ordinary skill in the art.

The dimming module 16 includes a voltage regulator 50 (e.g., a conventional regulator, such as a MIC5205 available from Micrel) to regulate the 12 VDC input from the auxiliary power supply 46 (input via a connector 52) to a DC level (e.g., 10 VDC) appropriate for use by the other electrical components in the dimming module. The dimming module further includes a timer 54 (e.g., a MIC1555 available from Micrel), a pulse width modulator 56 (e.g., a MIC502 available from Micrel), a driver 58 (e.g., a MAX4429 available from Maxim), and a switch assembly 60 which includes a pair of switching transistors 62 and 64 (e.g., n-channel MOSFETS, such as IXFT26N50 available from IXFT) coupled to the auxiliary winding 38 via a connector 66.

The pulse width modulator 56 provides a pulse width modulated signal to the driver 58, which provides the power to drive the MOSFET switches 62 and 64 between conducting and non-conducting states. The gates 68 and 70 of the MOSFET switches 62 and 64 are connected to resistors 72 and 74, respectively, which prevent undesired oscillation of the switches 62 and 64. The other ends of the resistors 72 and 74 are connected to an output 76 of the driver 58. Diodes 78 and 80 are connected from the source to the drain of switches 62 and 64, respectively. The sources of the FET switches 62 and 64 are connected together and to signal ground through a resistor 82.

The driver 58 provides a pulse width modulated waveform at its output to transition switches 62 and 64 between conductive (i.e., the driver output is at a HIGH level which is at or exceeds the turn-on threshold voltage of the switches 62 and 64) and non-conductive states (i.e., the driver output is at a LOW level which is at or below the threshold voltage to turn off the switches 62 and 64). The MOSFET 62 or 64 which is switched to a conducting state upon application of a HIGH level signal is determined by the polarity of the voltage reflected across the auxiliary winding 38 by the primary winding of the coupling transformer to which the auxiliary winding is coupled. That is, when the polarity of the voltage across the auxiliary winding 38 is such that the voltage at the drain 82 of the MOSFET 62 is positive with respect to the voltage at the drain 84 of the MOSFET 64 and a HIGH level signal is applied to the gates 72 and 74, the MOSFET 62 will transition to a conducting state. In this state, the MOSFET 64 is in a non-conducting state and a current carrying path is established through the auxiliary winding 38, through the MOSFET 62, and through the diode 80 and the internal parasitic diode (not shown) of MOSFET 64. Conversely, when the polarity of the voltage across the auxiliary winding 38 is such that the voltage at the drain 84 of the MOSFET 64 is positive with respect to the voltage at the drain 82 of the MOSFET 62 and a HIGH level signal is applied to the gates 72 and 74, the MOSFET 64 will transition to a conducting state. In this state, the MOSFET 62 is in a non-conducting state and a current carrying path is established through the auxiliary winding, through the MOSFET 64, and through the diode 78 and the internal parasitic diode (not shown) of MOSFET 62.

Thus, whenever either one of the MOSFETS 62 and 64 are in a conducting state, a short circuit is established across the auxiliary winding 38 which is reflected onto the primary winding of the coupling transformer 18 or 20 to which the auxiliary winding is coupled. As a result, the inductive coupling of energy to the electrodeless lamp 12 is interrupted, which substantially interrupts the generation of light and the flow of the AC lamp current. Accordingly, the average light generated by the lamp 12 during one cycle of the switching frequency of the switch assembly 60 can be adjusted by varying the time that the switches 62, 64 are in a conducting state (i.e., the duty cycle) during that cycle.

The switching frequency of the switch assembly 60 is set by a capacitor 86 connected to an input 88 of the pulse width modulator 56 (“PWM”). The capacitor 86 cooperates with internal components of the PWM 56 to create an oscillator which generates a repetitive ramp-shaped voltage signal at the input 88 of the PWM. The repetition rate of the ramp-shaped voltage signal corresponds to the switching frequency of the switch assembly 60. The duty cycle at which the switches 62 and 64 are driven is determined by the PWM's comparison of a variable amplitude voltage signal applied at an input 90 of the PWM 56 with the ramp at the input 88. In one embodiment, the amplitude of the voltage signal is determined by a voltage divider circuit that includes a brightness adjustment device (i.e., a potentiometer 92) which is connected to the dimming module via a connector 94. The value of the potentiometer 92 is selected such that the amplitude of the voltage signal at the input 90 of the PWM can be varied within a range that results in a duty cycle that is fully adjustable between 0% and 100%. The brightness adjustment device advantageously is accessible to a user of the dimmable light source such that the user can select a desired brightness level of the light generated by the electrodeless lamp. For example, the brightness adjustment device can include a control device (e.g., an adjustment knob) mounted on a panel of an enclosure containing the lamp system or mounted in any location accessible by a user of the light source. Alternatively, the brightness adjustment device may be an automatic device that automatically adjusts the brightness in response to a detected parameter, such as detection of ambient lighting conditions.

In certain embodiments, it may be desirable to limit the minimum duty cycle produced at the output of the PWM 56 to ensure a minimum time interval during which the switches 62 and 64 are not in a conducting state. In one embodiment, a voltage divider comprising resistors 96 and 98 is connected to an input 100 of the PWM 56 to ensure that the minimum duty cycle is limited to approximately 1%.

It may also be desirable to disable operation of the dimming module 16 for a brief time period (e.g., a few seconds) after the electrodeless lamp 12 is initially energized. To disable operation temporarily, the dimming module 16 includes the timer 54. Energization of the electrodeless lamp 12 is accompanied by energization of the 12 VDC auxiliary power supply 46 which provides power to the dimming module 16 via the connector 52. When auxiliary power is applied to the dimming module 16, the rate of rise of the voltage signal applied to the input of the timer 54 is controlled by the charging of a capacitor 102 (e.g., 4.4 uF) through a resistor 104 (e.g., 845 Kohms). During the time the voltage on the capacitor 102 is below a threshold level, the voltage produced at the output of the timer 54 remains at a HIGH level. The output of the timer 54 is coupled to the input 100 of the PWM 56 through a diode 106. While the timer output is HIGH, the diode 106 is forward biased, thus allowing application of the HIGH level voltage to the input 100 of the PWM 56, which prevents switching of the transistors 62 and 64, thus disabling dimming of the light generated by the lamp 12. When the voltage on the capacitor 102 exceeds the threshold level, the voltage at the output of the timer 54 transitions to a LOW level, thus reverse biasing the diode 106 which enables operation of the PWM 56 and, consequently, dimming of the light generated by the lamp.

In the illustrated embodiment, the switching frequency of the switch assembly 60 of the dimming module also can be adjusted via a frequency adjustment device 48 connected to the dimming module 16 via a connector 108. In one embodiment, the frequency adjustment device can be a potentiometer which can be varied to adjust the amount of current drawn by a constant current source (sink) connected to the input 88 of the PWM 56. The constant current source (sink) includes transistor 110 (e.g., a PNP transistor), transistor 112 (e.g., a NPN transistor), and resistors 114 and 116 (e.g., 100K ohms and 43K ohms, respectively). As the frequency adjustment device 48 is adjusted to increase the amount of current pulled by the transistor 112, the repetition rate of the ramp-shaped voltage signal at the input 88 of the PWM 56 decreases (i.e., the switching frequency of the dimming module decreases). Conversely, as the frequency adjustment device 48 is adjusted to decrease the amount of current sourced by transistor 112, the repetition rate of the ramp-shaped voltage signal increases. As discussed above, the frequency adjustment device 48 advantageously is accessible to a user of the dimmable light source such that the user can adjust the switching frequency to eliminate undesirable visual artifacts perceived in the lighting or on a display.

In an alternative embodiment, the frequency adjustment device 48 can be an electrical circuit configured to receive an electrical synchronization signal and to cooperate with the dimming module electronics to automatically synchronize the switching frequency to the received synchronization signal. For example, the frequency adjustment device 48, together with the constant current source (sink), can be configured as a phase-locked loop. That is, the frequency adjustment device 48 can be configured as a phase comparator that receives as an input the vertical video refresh signal from the video circuitry of a display unit which incorporates a dimmable electrodeless lamp system for backlighting. The frequency adjustment device 48 outputs a square wave voltage signal based on the comparison that causes the constant current source (sink) to synchronize the PWM oscillator. The phase-locked loop thus can synchronize the switching frequency of the switch assembly 60 to the frequency of the vertical refresh signal.

Turning now to FIG. 5, the assembly of the dimmable light source 10 is illustrated, including the electrodeless lamp 12, the coupling transformers 18 and 20, the auxiliary winding 38, the resistor 36, the dimming module 16, and the ballast 14. The cores of the coupling transformers 18 and 20 are separable into halves such that the transformers 18 and 20 can be removably secured to the lamp 12 by retaining spring clamps 118 and 120. The spring clamps 118 and 120 are further coupled to mounting brackets 122 and 124 for mounting the electrodeless lamp 12 in an appropriate housing.

A first end of the primary winding 30 (not shown in FIG. 5) of the coupling transformer 20 is connected in series with the resistor 36 via mating connectors 126 a and 126 b. The series combination of the resistor 36 and the primary winding 30 are connected in parallel with the primary winding 28 (not shown in FIG. 5) of the coupling transformer 18. The parallel connection points are connected to the ballast 14 via the mating connectors 128 a and 128 b. The secondary windings 32 and 34 of the coupling transformers 18 and 20 are the glass lamp vessel itself.

In one embodiment, the primary winding 30 of the coupling transformer 20 is wound within a 30° sector on the core 26. To maximize the coupling between the auxiliary winding 38 and the primary winding 30, the auxiliary winding is wound such that two turns are adjacent one end of the sector wound primary winding and the remaining two turns are adjacent the other end of the sector. The auxiliary winding can be wound onto the core of the transformer 20 simply by threading the wire through the core while the transformer 20 is positioned on the electrodeless lamp 12. Alternatively, the transformer 20 can be removed from the lamp 12 by removing the retaining spring clamp 120. The auxiliary winding 38 can then be appropriately wound onto the core and the core and spring clamp replaced on the lamp. Each end of the auxiliary winding 38 is connected to connection terminals 66 b.

The connection terminals 66 b mate with connection terminals 66 a which are connected to the dimming module 16 via a wire harness 130. Wire harnesses 132 and 134 also are connected to the dimming module 16 and terminate in connectors 94 a and 108 a. The connectors 94 a and 108 a are coupled to the brightness adjustment potentiometer 92 via the connector 94 b and the frequency adjustment potentiometer 48 via the connector 108 b, respectively.

In an exemplary embodiment, the dimmable light source assembly 10 illustrated in FIG. 5 can be incorporated in a display device assembly, such as the backlit display device assembly 136 illustrated in FIG. 6, which may be combined with other functional elements as illustrated in FIG. 7. The display device assembly 136 includes a display unit 138 having a conventional liquid crystal display (LCD) element 140 to display an image. The LCD element 140 typically is driven by a video driver 200 that generates appropriate electrical drive signals in response to input signals from a video source. In some embodiments, the video source may be the processing and control elements 202 of a computer. The front of the LCD element 140 is typically protected by a transparent screen 142 made of glass, plastic, or other suitable material. The screen 142 is mounted within an opening on a front frame 144 of the display unit 138 such that a user may clearly view the image displayed by the LCD element 140.

The display device assembly 136 also includes a diffuser 146 disposed adjacent the LCD element 140. The diffuser 146 is arranged to receive light generated by the lamp 12 and to transmit the light to the LCD element 140 such that the LCD element 140 is substantially uniformly illuminated.

The lamp 12 is mounted in a lamp housing 148 via mounting brackets (not shown), such as the mounting brackets 122 and 124 illustrated in FIG. 5. The lamp housing 148 includes a back reflective surface 150 and two end plates 152 and 154 which may be coated with a reflective material, such as an aluminum or silver-based material, or which may be made of a reflective material to form a reflective lamp lining. The lamp housing 148 can be formed, or stamped, from a suitable material in any number of well-known manufacturing processes. The reflective lamp lining may be configured to reflect light generated by the lamp 12 in a uniform manner toward the diffuser 146 and the LCD element 140.

The ballast 14, the dimming module 16 (not shown in FIG. 6), and the resistor 36 (not shown in FIG. 6) can be mounted to the interior or exterior surfaces of the lamp housing 148 in an appropriate manner, such as by using mounting brackets, mounting bosses, etc. The wiring harnesses 132 and 134 connecting the brightness adjustment potentiometer 92 and the frequency adjustment device 48 to the dimming module 16 can be appropriately routed such that devices 92 and 48 can be mounted to, for example, the front frame 144 of the display unit 138, or in any other location that is accessible to the user. Further, in some embodiments the dimming module may be connected to video display electronics (e.g., the video driver 200 or video source) to receive, for example, a video refresh signal to synchronize the switching frequency. Alternatively, the dimming module 16 may be configured to receive input signals from a processor or other control elements (e.g., the controller 202) of a computing device with which the display device assembly 136 is used.

The display device assembly 136 further includes a fan 156 mounted to the lamp housing 148 to cool the assembly. To provide further cooling, a heat absorbing sheet of glass 158 can be disposed in front of the lamp 12. Because the lamp 12 provides a bright light, the glass 158 can be relatively inefficient at transmitting light. A back casing 160 attaches to the front frame 144 to enclose the various components.

When the electrodeless lamp is used as a backlight source for a display assembly 136, artifacts, such as horizontal darkened bars or noise stripes, may be visible on the display element 140 of the display unit 138. As discussed previously, the appearance of such artifacts may be minimized by synchronizing the frequency of the dimming circuit to the vertical refresh frequency of the display unit or a multiple of the vertical refresh rate. For example, it has been found that for a refresh frequency of 60 Hz, a particularly advantageous synchronized dimming circuit frequency is 120 Hz.

It has been found that the presence of visible artifacts may also be countered by the use of a magnetic shield device 204 to oppose a stray magnetic field generated by the electrodeless lamp 12. As discussed above, an AC lamp current flows through the lamp 12 when the lamp is energized. The AC lamp current produces a time-varying magnetic field having a vector 203 at the center which is directed generally perpendicular to a plane 205 of the lamp 12 (see FIG. 9). The magnitude of the vector 203 decreases as the distance from the source of the magnetic field (i.e., the AC lamp current flowing in the lamp 12) increases. In an exemplary embodiment, the power source or ballast 14, which provides power to energize the lamp, operates at a frequency of 250 KHz. Accordingly, the time-varying magnetic field induced by the AC lamp current also has a frequency of 250 KHz.

When the lamp 12 is in close proximity to a display unit 138, such as when the lamp is used as a backlight source for the display unit, the stray magnetic field induces an electromotive force or voltage that may result in visual artifacts that appear on the display unit 138. These artifacts are particularly noticeable when the lamp 12 is dimmed. As described above, the video display assembly 136 may include the dimming module 16 to dim the lamp by pulse width modulating the energy provided to the lamp. Thus, the AC lamp current flows in bursts and induces corresponding bursts of the stray magnetic field, which induces corresponding bursts of an electromotive force or voltage. If the dimmed lamp 12 is in close proximity to a display unit 138, the visual artifacts on the display unit resulting from the induced electromotive force may appear as horizontal darkened lines or stripes. Although the appearance of such artifacts may be diminished by synchronizing the switching frequency of the dimming circuit 16 with the vertical refresh frequency of the display unit 138 as previously discussed, the artifacts still may be noticeable and detract from the viewing of an image on the display unit 138.

The magnetic shield 204, when placed on or in close proximity to the lamp 12, further diminishes the appearance of the artifacts by reducing the magnitude of the stray magnetic field. The shield 204, which operates in accordance with Lenz's law for induced current, is configured as a closed loop conductor through which current may flow. When the shield 204 is placed in the vicinity of the time-varying stray magnetic field resulting from the AC lamp current, the time-varying magnetic field induces a shield current in the closed loop conductor of the shield 204. The shield current, in turn, generates a magnetic field, which, in accordance with Lenz's law, is such that the change in magnetic flux due to the shield current is opposite to the change in magnetic flux that induced the shield current. Thus, the magnetic field generated from the shield 204 current may substantially oppose, or cancel, the stray magnetic field from the lamp 12. As a result, the induced electromotive force or voltage at the display unit 138 may be substantially eliminated or decreased such that the appearance of the visual artifacts on the display unit is substantially eliminated or diminished.

It should be noted that the magnetic shield 204 opposes the stray magnetic field generated by the AC lamp current regardless of whether the lamp 12 is being dimmed. The dimming mode is discussed in the exemplary embodiment solely to illustrate a situation in which the visual artifacts caused by the stray magnetic field are particularly noticeable.

In one exemplary embodiment, a closed loop magnetic shield is placed in close proximity to the electrodeless lamp 12 (e.g., on a side surface 206 or 208 of the lamp 12 or up to approximately 0.5 inches from the side surface of the lamp 12) between the lamp 12 and the display unit 138. In this embodiment, it has been found that the magnitude of the voltage induced by the stray magnetic field, when measured at the center of the display unit 138, may be decreased by approximately 50%. In this exemplary embodiment, the magnetic shield 204 is formed from a copper wire conductor and configured in a closed loop having a rectangular shape that approximates the rectangular dimensions of the electrodeless lamp 12. The magnetic shield is fitted over an outer surface of the lamp on a side 206 that will face the back surface of the display unit 138 when assembled therewith. The shield 204 may be held in position by the transformer spring clamps 118 and 120 or the mounting brackets 122 and 124, for example. Alternatively, the shield 204 may be adhered to the outer surface of the lamp 12 or the outer surface of the clamps 118 and 120 or brackets 122 and 124 by an appropriate adhesive (e.g., tape).

A partially exploded perspective view of an exemplary embodiment in which the shield 204 is shown detached prior to adherence to the outer surface of the clamps 118 and 120 by strips of adhesive tape 207 is illustrated in FIG. 5A. In this embodiment, the shield 204 is positioned for assembly proximate the outer surface of the lamp 12 on the side 206 which will face the back surface of a display unit 138 when assembled therewith. Mounting brackets 122 and 124 cooperate with mounting standoffs 123 and 125 such that the lamp assembly may be secured in an appropriate housing. Alternatively, the shield 204 may be disposed proximate an outer surface of the lamp on a side 208 (see FIG. 5) which does not face the back surface of the display unit 138. Other securing and positioning arrangements also are contemplated which may be appropriate to situate the magnetic shield 204 optimally such that it may substantially oppose the lamp magnetic field, thus reducing its affect on the display unit 138.

In another exemplary embodiment, the magnetic shield 204 may be configured as a multi-turn coil coupled in series with a primary of a coupling transformer, as illustrated in the schematic of FIG. 8, and placed in close proximity to the lamp 12. Because of the series connection (represented by a dashed line 210 in FIG. 7), the current waveform that flows through the magnetic shield 204 substantially replicates the AC lamp current. Thus, by appropriately selecting the number of turns of the coil of the magnetic shield 204, the stray magnetic field produced by the electrodeless lamp 12 may be substantially neutralized.

FIG. 8 illustrates an exemplary embodiment including a dimming circuit 16 to dim the light generated by the lamp 12. The dimming circuit 16 drives the auxiliary winding 38, which is electromagnetically coupled to the primary winding 30 of the coupling transformer 20. To dim the light generated by the lamp 12, the dimming circuit 16 provides a short circuit across auxiliary winding 38, which, in turn, is reflected as a short circuit across the primary winding 30, thus interrupting the energy provided to the lamp 12. Because a short circuit is provided across the primary winding 30 to dim the light, the shield 204 is connected in series with the primary winding 18 of the coupling transformer 28 such that the magnetic field generated by the shield 204 may oppose the magnetic field generated by the lamp 12 when the lamp is energized. In the exemplary embodiment illustrated in FIG. 8, when using an electrodeless lamp operating at 250 KHz, 100 W, it has been found that the induced voltage measured at the center of the LCD panel 140 may be substantially eliminated by a magnetic shield 204 made of 24 AWG copper wire and having nine turns. This magnetic shield 204, which is coupled in series with the primary winding 18 of the coupling transformer 18, is formed in the shape of a rectangle and positioned proximate an outer surface of the lamp 12 on the side 206 of the lamp 12 which will be facing the display unit 138 when assembled therewith. Alternatively, the shield 204 may be placed on the side of the lamp 12 opposite the side which will face the display unit 138. The shield 204 may be secured in a manner similar to that described above with respect to the closed loop configuration of the magnetic shield 204.

FIG. 10 illustrates another exemplary embodiment of a dimmable electrodeless light source in which the magnetic shield 204 reduces visual artifacts that appear on a display unit when the electrodeless lamp 12 is dimmed. In this embodiment, the shield 204 is disposed proximate the lamp 12, as described above, and is coupled in series with the auxiliary winding 38, which is driven by the dimming circuit 16. In this embodiment, the magnetic field generated by the shield 204 compensates for the effects of the magnetic field generated by the lamp 12 by maintaining a substantially constant total magnetic field at all times rather than by opposing the magnetic field generated by the lamp when the lamp is energized. By connecting the shield 204 in series with the auxiliary winding 38, the shield 204 produces a magnetic field during the portions of the dimming cycle in which the current flow in the lamp 12 is interrupted or reduced. The shield 204 preferably is configured such that the magnetic field generated by the shield 204 combines with the lamp magnetic field to result in a total magnetic field proximate the display unit 138 that is substantially constant at all times. Thus, for example, if the lamp magnetic field is substantially eliminated when the auxiliary winding 38 provides a short circuit across the primary winding 30 of the coupling transformer 20, then the shield device 204 may be configured to generate a shield magnetic field that has substantially the same magnitude and direction as the magnetic field that was being generated by the lamp 12. As a result, the total magnetic field proximate the display unit 138 is substantially constant at all times.

Alternatively, the dimming circuit may be configured to reduce the level of energization during a portion of the dimming cycle, which causes the magnitude of the lamp magnetic field to be reduced rather than substantially eliminated. In such an embodiment, the shield device 204 may be configured to generate a shield magnetic field to compensate for the reduction of the lamp magnetic field during the dimming portion of the dimming cycle. As a further alternative, the shield device may be configured to generate a magnetic field at all times during an operational mode of the electrodeless lamp 12 in which the amount of energy provided to the lamp may vary. In such an embodiment, the shield magnetic field also may be varied to compensate for changes in the lamp magnetic field that may occur due to changes in the level of energization of the lamp 12. As a result, the total magnetic field may be held substantially constant during the entire operational mode, regardless of any variations in the energization level of the lamp 12.

It should be understood that the dimmable video display assembly illustrated in the FIGURES is merely an exemplary application of the dimmable light source. Other applications for the shielded light source can be readily envisioned, such as lighting systems or light fixtures for the home or office in which incorporation of the magnetic shield 204 may also be beneficial. Further, it is envisioned that the assembly may include other components and mounting arrangements depending on the application and the intended use of the display, as would be realized by a person of ordinary skill in the art.

While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims. 

What is claimed is:
 1. A backlit display device, comprising: an electrodeless lamp to generate light; a power source coupled to the electrodeless lamp to energize the lamp, wherein an AC lamp current flows through the lamp when the lamp is energized, the AC lamp current producing a lamp magnetic field; a display unit to display an image, the electrodeless lamp being operably positioned with respect to the display unit to illuminate the display unit to enhance viewing of the image; and a shield device operably positioned with respect to the lamp and the display unit to reduce an effect of the lamp magnetic field on viewing of the image on the display unit.
 2. The backlit display device as recited in claim 1, comprising: a dimming module coupled to the electrodeless lamp, the dimming module being configured to interrupt flow of the AC lamp current to adjust brightness of the generated light.
 3. The backlit display device as recited in claim 2, wherein the dimming module interrupts the flow of the AC lamp current at a repetition rate.
 4. The backlit display device as recited in claim 3, comprising: a computer; and a video driver coupled to the computer and to the display unit to control display of the image and to refresh the image at a refresh rate, wherein the repetition rate of the dimming module is synchronized to the refresh rate.
 5. The backlit display device as recited in claim 1, wherein the shield device comprises a conductor configured in a closed loop.
 6. The backlit display device as recited in claim 5, wherein the closed loop conductor is positioned between the electrodeless lamp and the display unit.
 7. The backlit display device as recited in claim 1, wherein the shield device comprises a conductive coil having a plurality of turns, the coil being coupled between the power source and the electrodeless lamp.
 8. The backlit display device as recited in claim 7, wherein the power source comprises a ballast.
 9. The backlit display device as recited in claim 1, wherein the display unit comprises a liquid crystal display panel.
 10. A method for reducing an effect of a first magnetic field on a display unit, the method comprising: coupling energy to an electrodeless lamp to energize the electrodeless lamp, to generate light, and to generate the first magnetic field; restricting the coupling of the energy during a dimming portion of a dimming time period to dim the generated light; and generating a second magnetic field that substantially opposes the first magnetic field.
 11. The method as recited in claim 10, wherein generating the second magnetic field comprises generating a shield current in a shield device, the shield device being operably positioned with respect to the electrodeless lamp such that the second magnetic field substantially opposes the first magnetic field.
 12. The method as recited in claim 11, wherein the shield device is positioned between the electrodeless lamp and the display unit.
 13. The method as recited in claim 11, wherein the shield device comprises a conductor configured as a closed loop.
 14. The method as recited in claim 11, wherein the shield device comprises a conductive coil.
 15. The method as recited in claim 10, wherein an AC lamp current flows in the electrodeless lamp when the lamp is energized, and wherein restricting the coupling of the energy comprises interrupting flow of the AC lamp current during the dimming portion of the dimming time period.
 16. The method as recited in claim 15, comprising: adjusting a length of the dimming portion of the dimming time period to dim the generated light over a dimming range.
 17. The method as recited in claim 15, comprising: displaying an image on the display unit; refreshing the image on the display unit at a refresh frequency; and synchronizing a dimming frequency of the dimming time period with the refresh frequency.
 18. A method for making a magnetically shielded dimmable electrodeless light source for a backlit display device, the method comprising: providing an electrodeless lamp having a coupling transformer to provide energy to energize the lamp; disposing an auxiliary winding on the coupling transformer; coupling the auxiliary winding to a drive circuit, the drive circuit being configured to establish a current carrying path through the auxiliary winding for a first time interval of a time period and to interrupt the current carrying path through the auxiliary winding for a second time interval of the time period to control brightness of light generated by the lamp; and disposing a magnetic shield device proximate the electrodeless lamp, the magnetic shield device being disposed such that the magnetic shield device produces a shield magnetic field that substantially opposes a lamp magnetic field generated by the lamp when the lamp is energized.
 19. An electrodeless light source, comprising: an electrodeless lamp which, when energized, generates a lamp magnetic field, the electrodeless lamp operable in an operational mode comprising at least a first level of energization and a second level of energization, the second level being different than the first level; and a shield device disposed proximate the electrodeless lamp and configured to generate a shield magnetic field, which, when combined with the lamp magnetic field, results in a substantially constant total magnetic field during the operational mode of the lamp.
 20. The electrodeless light source as recited in claim 19, wherein the electrodeless lamp is substantially deenergized when at the second energization level.
 21. The electrodeless light source as recited in claim 19, wherein the operational mode comprises a dimming mode in which intensity of light generated by the electrodeless lamp is reduced.
 22. The electrodeless light source as recited in claim 19, comprising: a dimming circuit coupled to the electrodeless lamp to control the level of energization of the lamp.
 23. The electrodeless light source as recited in claim 22, wherein the dimming circuit is coupled to the shield device.
 24. The electrodeless light source as recited in claim 23, comprising a coupling transformer inductively coupled to the electrodeless lamp, the coupling transformer coupling a power source to the lamp to energize the lamp.
 25. The electrodeless light source as recited in claim 24, comprising an auxiliary winding electromagnetically coupled to the coupling transformer to couple the dimming circuit to the electrodeless lamp.
 26. The electrodeless light source as recited in claim 25, wherein the shield device is coupled in series with the auxiliary winding.
 27. The electrodeless light source as recited in claim 19, wherein the shield device comprises a conductive coil.
 28. A method for making a magnetically shielded dimmable electrodeless light source for a backlit display device, the method comprising: providing an electrodeless lamp having a coupling transformer to provide energy to energize the lamp; disposing an auxiliary winding on the coupling transformer; coupling the auxiliary winding to a drive circuit, the drive circuit being configured to establish a current carrying path through the auxiliary winding for a first portion of an operational mode and to at least partially obstruct the current carrying path through the auxiliary winding for a second portion of the operational mode to control brightness of light generated by the lamp; and disposing a magnetic shield device proximate the electrodeless lamp, the magnetic shield device being disposed such that the magnetic shield device produces a shield magnetic field that substantially compensates for changes in a lamp magnetic field generated by the lamp during the operational mode.
 29. A method for reducing an effect of a first magnetic field on a display unit, the method comprising: energizing an electrodeless lamp at a first energization level to generate light and to generate the first magnetic field; energizing the electrodeless lamp at a second energization level to dim the light, the second energization level causing a variation of the first magnetic field; and generating a second magnetic field that substantially compensates for the variation of the first magnetic field.
 30. The method as recited in claim 29, wherein energizing the electrodeless lamp at the second energization level comprises substantially deenergizing the lamp.
 31. The method as recited in claim 29, wherein generating the second magnetic field comprises generating a shield current in a shield device, the shield device being operably positioned with respect to the electrodeless lamp such that the second magnetic field substantially compensates for the change of the first magnetic field.
 32. An electrodeless light source, comprising: an electrodeless lamp which generates a lamp magnetic field; and a shield device disposed proximate the electrodeless lamp and configured to generate a shield magnetic field, which, when combined with the lamp magnetic field, results in a substantially constant total magnetic field.
 33. The electrodeless light source as recited in claim 32, comprising: a dimming circuit to dim light generated by the electrodeless lamp, wherein the lamp magnetic field varies when the dimming circuit dims the light.
 34. The electrodeless light source as recited in claim 33, wherein the shield device is coupled to the dimming circuit.
 35. An electrodeless light source, comprising: an electrodeless lamp which generates a variable lamp magnetic field; and a shield device disposed proximate the electrodeless lamp and configured to generate a variable shield magnetic field, the lamp magnetic field and the shield magnetic field varying opposite one another.
 36. The electrodeless light source as recited in claims 35, wherein a magnitude of the shield magnetic field varies opposite a magnitude of the lamp magnetic field.
 37. The electrodeless light source as recited in claim 35, wherein a direction of the shield magnetic field varies opposite a direction of the lamp magnetic field.
 38. The electrodeless light source as recited in claim 35, wherein the shield magnetic field varies such that the shield magnetic field substantially opposes the lamp magnetic field.
 39. The electrodeless light source as recited in claim 35, wherein the shield magnetic field varies such that, when combined with the lamp magnetic field, a substantially constant magnetic field is provided.
 40. A system for compensating for an effect of a variation of a first magnetic field on a display unit, the system comprising: means for energizing an electrodeless lamp to generate light and to generate the first magnetic field; means for varying energization of the electrodeless lamp, thereby varying the first magnetic field; and means for generating a variable second magnetic field to substantially compensate for the variation of the first magnetic field.
 41. The system as recited in claim 40, wherein the second magnetic field varies to substantially oppose the first magnetic field such that a combined total magnetic field is substantially zero.
 42. The system as recited in claim 40, wherein the second magnetic field varies such that, when combined with the first magnetic field, a combined total magnetic field is substantially constant. 